This invention relates to a process for making coated glass. In particular this invention relates to a process for making infra-red reflecting glass and the resulting infra-red reflecting glass.
There is an increasing demand in both the architectural and transport markets for glazings that have heat-insulating properties. The heat insulation of glazing panes may be improved by coating them with infra red reflecting coatings, also known as low emissivity coatings. Low emissivity coated panes have normal emissivities of about 0.2 or less, but lower emissivities are preferable, especially 0.1 or less. Normal emissivity (xcex5N) and the U value (which is a measure of the insulation of a glazing) are defined in International Standard ISO 12092: 1994 (F), Annex A, normal emissivity being xcex5N=1xe2x88x92RN, where RN is the normal reflectance at 283 K for wavelengths 5.5 xcexcm to 50 xcexcm.
Low emissivity coated glasses may comprise a coating stack incorporating at least one reflective metal layer (for example, of aluminium, copper, gold or preferably silver) sandwiched between anti-reflection layers of, for example, oxides of tin, zinc or titanium, silicon nitride or zinc sulphide. Coatings such as these are electrically conductive, the emissivity and resistance of the coatings being related in that an increase in resistance is usually associated with an increase in its emissivity, a more conductive coating having a better infra-red reflectivity.
To produce reflective metal low emissivity coatings on a large scale it is advantageous to use a process allowing a high deposition rate, for example, sputtering. For metal oxide anti-reflection layers, the highest deposition rates tend to be achieved by reactive sputtering from an electrically conductive (usually metallic) target in an atmosphere containing oxygen or an oxidising gas. During the sputtering process the oxygen component of the coating atmosphere is incorporated in the coating resulting in the deposition of a metal oxide coating. Unfortunately, when attempts are made to produce a coating comprising a metal oxide layer on top of, for example, a silver reflective metal layer by a reactive sputtering process in the presence of oxygen, the reflective metal layer can react with the oxygen resulting in loss of the low emissivity properties of the reflective metal layer. The reaction is not thought to be a simple oxidation because it may also involve agglomeration in the reflective metal layer.
The problem of oxidation of silver layers during overcoating with metal oxides by reactive sputtering in an oxidising atmosphere is described in United Kingdom patent application 2 129 831A. GB 2 129 831A also describes how this problem may be overcome by sputtering a small proportion of a metal other than silver before the metal oxide layer so that the additional metal lies predominantly over or in the upper part of the silver layer. The specification describes a process for the production of a low emissivity coating on a transparent substrate by cathode sputtering comprising, in sequence, sputtering a layer of silver from 5 to 30 nm thick, sputtering an additional metal other than silver in an amount equivalent to a layer 0.5 nm to 10 nm thick on to the silver layer and reactively sputtering, in the presence of an oxidising gas, an anti-reflection metal oxide layer over the silver and additional metal. In European patent specification EP 0 543 077B the problem of oxidation of a reflective metal layer during reactive sputtering of metal oxides is similarly addressed by deposition of a barrier layer. The barrier layer comprises a first sub-layer of platinum or palladium with a thickness in the range 0.2 to 0.5 nm adjacent to a silver or copper layer (the reflective metal layer) and a second sub-layer composed of titanium and/or chromium or an alloy with a thickness in the range 0.5 to 5.0 nm. The barrier layer is overcoated with a tin/ytterbium oxide layer deposited by reactive sputtering in an argon-oxygen atmosphere. Coatings on glass substrates prepared by sputtering and having layers of silver and a metal oxide are not usually stable to heating in air. The coating can lose its properties of low sheet resistance (and hence low emissivity) when the coated glass is subjected to a thermal cycle required for bending or toughening the glass.
We have discovered that the problem of oxidation of a reflective metal layer during reactive sputtering of a metal oxide may be alleviated in accordance with the present invention by depositing the metal oxide in an atmosphere containing an oxygen scavenger. We have also discovered that coated glasses produced in such a way are less prone to losing their properties of low sheet resistance (and hence low emissivity) when subjected to heating.
According to an aspect of the invention there is provided a process for the production of an electrically conductive low emissivity coating on a glass substrate comprising, in sequence,
(a) depositing a reflective metal layer and
(b) depositing a metal oxide layer over the metal layer by reactive sputtering in the presence of an oxygen scavenger.
This is advantageous because fewer layers (i.e. no additional protective layer or layers) are required to be deposited than in the prior art processes.
The reactive sputtering is carried out, as is understood in the art, in the presence of oxygen or an oxidising gas, which reacts to form the metal oxide layer. Thus, preferably the metal oxide layer is reactively sputtered in an atmosphere containing the oxygen scavenger and an oxidising gas. Usually, the oxidising gas is oxygen.
The oxygen scavenger is advantageously a hydrocarbon, preferably a C1 to C4 hydrocarbon. More preferably the oxygen scavenger is an alkane, especially a C1 to C4 alkane, most preferably the oxygen scavenger is methane.
The invention enables the overlying metal oxide layer to be sputtered at an increased rate (as compared to the rate which can be achieved in the absence of the oxygen scavenger) while maintaining a required low emissivity (i.e. a normal emissivity of 0.2 or less).
The metal oxide may comprise tin oxide, zinc oxide or, preferably, tungsten oxide. Other metal oxides likely to be suitable include transition metal oxides, for example, tungsten oxide, nickel oxide, molybdenum oxide, manganese oxide, zirconium oxide, vanadium oxide, niobium oxide, tantalum oxide, and mixed oxides comprising these metal oxides. Cerium or titanium oxide may also be suitable. Some metal oxides (for example tungsten oxide) may be partially reduced during deposition of the metal oxide layer in the presence of an oxygen scavenger, such partial reduction may change the optical properties of the metal oxide layer.
The reflective metal layer will normally comprise a silver layer. This is advantageous because silver layers are highly reflective in the infra-red region of the spectrum, have a neutrally coloured visible reflection and are easily deposited by sputtering, or other vacuum deposition methods.
Low emissivity coatings comprising reflective metal layers produced by a process according to the invention will normally undergo an increase of sheet resistance on overcoating with the metal oxide layer of less than 25%, and preferably less than 20%. Usually, the coatings produced by a process according to the invention will have a sheet resistance after overcoating with the metal oxide layer of below 12 xcexa9/square. This is advantageous because sheet resistances below 12 xcexa9/square indicate that the coating and, therefore, the coated glass, has low emissivity acceptable for commercial purposes.
A particular advantage of the present invention is that coated glasses produced by the process according to the invention are less prone to degradation after heat treatment in an oxidising atmosphere (usually of air). Such heat treatment is necessary, for example, to shape or toughen glass.
Thus, in a preferred embodiment of the invention, after depositing the reflective metal layer and the metal oxide layer, the process additionally comprises heat treating the coated glass substrate in an atmosphere containing oxygen.
Preferably, the sheet resistance of the coating is below 30 xcexa9/square after the glass has been overcoated with the metal oxide layer and then heat treated in an atmosphere containing oxygen. Sheet resistance of below 30 xcexa9/square after heat treatment indicates that the reflective metal layer, although partially oxidized, is not completely oxidized, but preferably the sheet resistance would be lower, and more preferably about the same as before heat treatment. The coated glass would be heat treated if it is desired to shape and/or toughen the glass after coating. The heat treatment would usually involve raising the temperature of the glass to 400-720xc2x0 C.
The thicknesses of the reflective metal layers in coated glasses produced by a process according to the invention will normally be in the range 8 to 18 nm. Thicknesses below 8 nm may result in too high an emissivity (i.e. too little infra red reflection), above 18 nm there may be high visible light absorption by the reflective metal layer.
If it is desired to produce coated glasses having very low emissivity or having solar control (i.e. lower total solar heat transmission) properties more than one reflective metal layer may be deposited sandwiched between anti-reflection layers, (i.e. the coating may comprise 2n+1, anti-reflection and reflective metal layers, where n is an integer greater than or equal to one).
If it is desired to produce a toughenable coated glass (i.e. protected from degradation during heat treatment or ageing), the coating stack may comprise an additional protective layer disposed between the reflective metal layer and the overcoating metal oxide layer. Useful protective layers include zinc, titanium, nickel, palladium, platinum or preferably a nickel chromium alloy.
Coated glass according to this invention may be used in many areas of glass use including in laminated glass comprising a coated glass according to the invention, a polymer interlayer and a second glass and in multiple glazing units having a first pane of a coated glass in spaced opposed relation to a second glass.
In another aspect, the present invention also provides a low emissivity coated glass comprising a glass substrate and a coating stack deposited directly on the substrate said coating stack comprising a silver layer and at least one tungsten oxide layer over said silver layer.